Delivery of high pulsed laser power through optical fibers is widely used for ablation of tissue or other targets. For such ablation procedures, ultra-violet (UV) light has many advantages, as it is well absorbed by biological matter and organic compounds. Rather than burning or cutting material, the UV laser adds enough energy to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the air in a tightly controlled manner through ablation rather than burning. The laser energy is also strongly absorbed and leads to sharp local elevation of temperature and results in generation of strong mechanical forces leading to photo-acoustic and photo-thermal ablation. Thus lasers emitting in the ultraviolet have the useful property that they can remove exceptionally fine layers of surface material with almost no heating or change to the remainder of the surrounding material which is left intact. Excimer lasers emitting at 308 nm (XeCl) are commonly used. However, such lasers are bulky, require careful maintenance and frequent calibration, and the beam quality is poor and may not be stable. Third harmonic, Q-switched Nd:YAG lasers emitting at 355 nm have also been used for such UV ablation procedures.
In order to obtain effective tissue ablation, fluencies above a certain threshold are required, and high peak power pulses, of the order of 50 mJ/mm2 in pulses of down to the 10 nsec range are generally desired. The delivery of such fluences is very challenging for the optical fibers, and can lead to damage at the entrance or exit facets of the fiber, or in the bulk of the fiber by selective heating, plasma generation, self-focussing or the generation of cracks at the exit facet. In order to overcome the challenge of this kind of damage, methods have been proposed in the prior art of taking the high quality beam emitting by the laser, and of homogenizing the beam before input to the fiber, to eliminate “hot spots”. Some such methods that have been proposed include:
(i) The laser is coupled to the optical fiber using a diffractive optical element (DOE) and a coupling lens. The DOE homogenizes the beam spatial energy density and eliminates “hot spots” of the laser. The DOE can form different shapes, including a square, a circle, rectangular, as adapted to the different shapes of the input plane of a bundle of fibers. Different coupling lenses and different distances between the DOE and the coupling lens have been used, in order to obtain different spot sizes.
(ii) A micro-lens array is used to achieve a homogenized spot. More than one array can also be used in order to achieve better homogeneity and to avoid hot spots due to beamlet interference. The size and shape of the spot can be manipulated by varying the pitch size and the coupling lens focal length. Such a micro-lens array homogenizer for executing the coupling of high peak power laser pulses into optical fibers, has been described in an article by T. Schmidt-Uhlig et al, entitled “New Simplified Coupling Scheme for the Delivery of 20MW Nd:YAG Laser Pulses by Large Core Optical Fibers”, published in Applied Physics B, Lasers and Optics, Vol. 72, pages 183-186 (2001).
(iii) Use of a multimode fiber in order to homogenize the beam energy density. A positive lens is used to couple the laser beam into the homogenizer fiber, and a second positive lens is used to image the homogenizer fiber output into the fiber delivering the beam to the ablation target. A convenient option is to use a fused silica fiber, which is more suitable for high power transmission.
(iv) The use of pairs of pulses to achieve effective removal of tissue from a surgical site, in which the first pulse “conditions” the tissue which can then be more easily removed by a second, often longer pulse. This arrangement enables ablation to the accomplished with less damage to the fiber than if an equivalent single pulse were to be used. Such a method is described in U.S. Pat. No. 5,312,396 for “Pulsed Laser System for the Surgical Removal of Tissue” to M. S. Feld et al.
Additionally, a similar procedure using multiple pulses is described in the article by D. Albagli et al entitled “Time dependence of laser-induced surface breakdown in fused silica at 355 nm in the nanosecond regime”, published in SPIE Vo. 1441, Laser induced Damage in Optical Materials, 1990. Using first and second pulses of the pair at two different wavelengths may also be advantageously used.
In an alternative approach, the pulse length of the laser has been extended to more than 100 nsec in order to improve the damage threshold of the fibers, or has been split into at least two pulses with a 100 to 200 nsec delay between them, but this comes at the expense of the ablation efficiency of hard tissues, such us highly calcified lesions as described in the article by Rod S. Taylor et al entitled “Dependence of the XeCl laser cut rate of plaque on the degree of calcification, laser fluence, and optical pulse duration” published in “Lasers in Surgery and Medicine” Volume 10, Issue 5, pages 414-419, 1990.
However, all of the above mentioned methods have disadvantages, particularly in terms of the limited improvement in energy density carrying capacity that can be achieved for the optical fiber setup used, and/or the system energy throughput, and/or damage to the fiber tip when in contact with tissue.
There therefore exists a need for a method and apparatus for performing ablative surgical methods using fiber optical delivery of the ablation energy, which overcomes at least some of the disadvantages of prior art systems and methods.
In addition to the need for new systems for enabling the ablation process, there is a growing need for the specific procedure of removing pacemaker and defibrillator leads in patients, due to such reasons as lead fracture or abrasion of the insulation causing shorting and infections. Approximately 5 million leads are implanted worldwide and it is estimated that 4-7% will have to be removed at some time during the patient's lifetime. It is estimated that over 100,000 leads were extracted in the US and Europe in 2010.
In Lead Extraction procedures, known hereinafter as LE, the most critical point in the procedure is reached when the lead at a bend in the vein has to be debulked. When the electrode separation procedure is performed, there is a risk of perforation of the vein by the catheter, and in severe cases, this can even result in death of the patient. Rates of 1% death cases or even higher are reported using active dilators.
Laser ablation and mechanical based cutters are widely used solutions for atherectomy procedures in order to open or partially open blockages inside blood vessels. One of the methods of reducing the danger of the vessel wall perforation is by using a system having parameters which preferentially cut or ablate the atheroma tissue over the wall of the blood vessel. If the cutting or ablating effect is significantly more effective on the atheroma material than on the artery or vein wall material, and the procedure is executed under conditions which fall safely below the threshold at which damage may be caused to the vessel wall, there will be less likelihood that the artery or vein wall will be cut during the debulking procedure. In the prior art, in an article entitled “Preferential Light Absorption in Atheromas in Vitro—Implications For Laser Angioplasty” by M. R. Prince et al, published in Journal of Clinical Investigation Vol. 78(1): pages 295-302, July 1986, it has been shown that atheromas indeed absorb more than the normal aorta between 420 and 530 nm. However, this was not found to be so in the UV, where at the widely used 308 nm wavelength, the absorption by the aorta is higher than that of atheroma. However, since use of the 420-530 nm range, with its advantageous ablation selectivity, has an inherent disadvantage in the potential thermal damage caused by the larger energies needed for efficient ablation and deeper penetration, it is preferable to use a method for selective ablation which uses laser radiation within the UV region.
However, it has also been found, as described in the article entitled “Laser Ablation of Atherosclerotic Blood Vessel Tissue Under Various Irradiation Conditions” by R. O Esenaliev et al, published in IEEE Transactions on Biomedical Engineering. Vol. 36, no. 12. Pages 1188 to 1194 (December 1989), that for wavelengths in the UV (355 nm and 266 nm), no difference in the optical attenuation coefficients to short pulses, has been found between the normal wall and fibrous plaque areas of atherosclerotic blood vessels. Therefore, other prior art methods, such as shown in the article entitled “Selective Absorption of Ultraviolet Laser Energy by Human Atherosclerotic Plaque Treated with Tetracycline” by D. Murphy-Chutorian et al, published in the American Journal of Cardiology, Vol. 55, pages 1293-1297, 1985, have suggested the use of sensitizers such as tetracycline, to increase the absorption in the plaque. Tetracycline binds strongly to the plaque and has strong absorption in the UV. The problem with such methods for use in clinical treatments is that tetracycline is an antibiotic, and needs additional regulation and tests to ensure absence of side effects.
There therefore also exists a need for a method and apparatus for safely performing lead extraction, using fiber optical delivery of the ablation energy, which overcomes at least some of the disadvantages of prior art systems and methods. Similarly, there is a need for atherectomy tools for debulking of atheroma in blood vessels that reduce the risk of vessel perforation or dissection and debulking of enlarged glands in Benign Prostatic Hyperplasia (BPH) while reducing the risk of capsule injury.
Laser catheters should be calibrated prior to the operation in order to verify the fluence and the repetition rate of the laser energy that is emitted from the catheters.
The prior art deals with methods of calibration of catheters in which the catheter is pulled out of its packaging, coupled to the laser system, the distal tip is held by a housing in front of a detector, the laser is operated and the energy is measured by the detector as described in U.S. patent Ser. No. 11/946,376 for “Laser catheter calibrator” to Tom Dadisman.
Since the catheters are sterilized before use, this method can involve risk of moving the distal tip of the catheter out of the sterilized area in the operation room.
There therefore also exists a need for a method and apparatus for internal calibration of the laser system and for detecting a failure of the system and/or the catheter.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.